1. Field of the Invention
The present invention relates to the manufacture of electronic components, and more particularly to forming recessed patterns such as trenches and vias in insulating substrates such as polymers and ceramic green sheets by depositing a castable insulator into a mold.
2. Description of Related Art
Smaller and faster electronic devices offer obvious advantages. Fast electronic devices usually require short connections between the attendant integrated circuit chips. This in turn demands that the chips be located close together. Close chip spacing also provides for smaller devices. To locate chips close together they are commonly mounted on the top layer of a flat, insulating substrate and interconnected by conductors routed in several underlying layers of the substrate. This arrangement is called a multi-chip module ("MCM").
In producing a typical MCM, conductors are applied to a single 4 or 5 mm thick layer of ceramic substrate and each layer of the substrate-conductor combination is then stacked with dozens of other layers. In some methods the conductors are routed on the surface of each layer, while in others the conductors are embedded or recessed into each layer. Ceramic is well suited for substrates because it has low thermal expansion, high thermal conductivity, mechanical strength, excellent electrical insulation, and reasonable cost. However, it may be difficult to form the recessed patterns needed for embedded conductors in ceramic because ceramic is dense even before it is fired. After it is fired ceramic is very hard and brittle and thus difficult to pattern.
A ceramic substrate is typically produced by first forming an unpatterned mold or "carrier tape" such as by extruding a thermal plastic film. A suspension of aluminum particles in a polymer binding is then cast onto the carrier tape. This suspension is then cured to a patternless unfired sheet ("ceramic green sheet") to be used as one substrate layer in a multilayer MCM. After conductors are applied to each layer, the layers are stacked together and fired into a rigid unit.
Several methods are known for forming patterns of conductors in layers of ceramic substrates without first forming recesses therein. Typically these methods extrude the conductors onto the substrate through a metal mask by screen printing. But screen printing cannot produce conductors with dimensions that are high relative to width, that is, it cannot widely vary the conductor aspect ratio by varying conductor height. Nor can screen printing produce conductors of precise dimensions. Furthermore, screen printing does not allow the layers of substrate and conductors to be tightly stacked before firing because they are not entirely flat.
Several methods are also known for improving flatness where the conductors are formed on the substrate surface. One such method, disclosed in U.S. Pat. No. 4,109,377 issued to Blazick et al., places the conductors on the surface of each layer of the substrate and presses the layers together, thereby somewhat flattening the conductors. Also, the conductors cannot be closely spaced because pressing the layers together causes the conductors to spread out. This conductor spreading also limits the signal propagation speed as described in U.S. Pat. No. 4,581,098 issued to Gregor. Another method, disclosed in U.S. Pat. No. 4,825,539 issued to Nagashima et al., presses conductors into the top of each layer while cooling the conductors and heating the substrate. This does result in a flat surface. It also reduces, although does not entirely eliminate, spreading of the conductors. Besides not altogether eliminating conductor spreading, a further disadvantage is the heating and cooling required. Another method, disclosed in U.S. Pat. No. 5,009,744 issued to Mandai et al., reduces the thickness of the metal layer on the surface of the substrate by forming it first on a "back film" and then transferring it to the substrate. This method does not, however, produce an entirely flat surface.
Another method forms a partial pattern of recesses in the surface of the substrates. U.S. Pat. No. 4,715,117 issued to Enomoto describes forming a substrate with a regular pattern of through-holes, typically by mechanical or laser drilling or by punching; then the unwanted holes are filled; finally the selected holes are metal plated at the same time that the surface of the substrate is plated. This method does not produce a flat surface because it does not provide a complete pattern of recesses. That is, it provides through-holes or "vias" for conductors that run between the flat surfaces of the substrate layer, but it does not provide "trenches" that are needed for the conductors that run parallel to the top major surfaces of the substrate. Besides that disadvantage, this method does not fill the entire through-hole with conductor material.
Other methods do provide a complete pattern of recesses. Recesses have commonly been formed by pressing a mold into a flat ceramic green sheet substrate. However, as the Gregor '098 Patent referred to above points out, the green sheet is too hard and dense for this "branding" technique to work well. For example, the features of the mold or punch must be relatively large in order to be sturdy enough to press a pattern into the green sheet. Consequently, other techniques have been developed to "thermally machine" the ceramic green sheet, such as by exposing the ceramic green sheet to laser or electron beam radiation through a mask. These thermal machining methods are effective but require expensive and delicate machinery applied under carefully controlled conditions. Another method, disclosed in the Gregor '098 Patent, forms a pattern of xeroxed, stenciled, or photolithographed lines on the substrate surface and then exposes the lined surface to a source that selectively heats the lines until the substrate underneath vaporizes. Gregor's method, however, has limited reproduceability due to inconsistent thermal decomposition.
In summary, prior methods of applying conductors to a substrate where the resulting substrate-conductor surface is not flat are relatively simple, but frequently do not allow stable, compact multilayered structures, precise conductor dimensions, or wide variation in conductor aspect ratios. The simplest methods to make the layers more stable and compact cause conductor spread, which limits the size, spacing, and resistivity of the conductors. These spacing and resistivity limitations prevent smaller packages and faster devices. By forming recessed patterns in the ceramic substrate the layers can be stacked tightly without spreading the conductors, but this requires additional steps, and sometimes requires expensive, delicate equipment operated under carefully controlled conditions. And some simple methods of producing recessed patterns are not capable of producing very small features.